Setting and resetting of level of postural muscle tone in decerebrate cat by stimulation of brain stem.

Abstract

The brain stem areas subserving setting and resetting of the extensor muscle tone related to the standing posture in acute precollicular-postmammillary decrerebrate (mesencephalic) cats are described. A microelectrode was inserted systematically into the brain stem with 0.5-mm steps (H-4 to H-11) at levels ranging from P3 to P8 rostrocaudally and from LRO to L2 or R2 mediolaterally. The changes in the level of postural muscle tone were estimated by measuring the force through the hindlimbs and the electromyograms (EMG) of the hindlimb muscles while the animal maintained standing posture with the aid of a rubber hammock. Stimulation of the dorsal part of the pons along the midline (P3 to P7, H-4.5 to H-6) decreased the tone of the hindlimb muscles. The decrease in the force and in tonic discharge of extensor muscles increased with an increase in stimulus intensity. Such a decrease in the force level continued for > 5 min after the stimulation. During this period the animal was able to maintain standing posture only with the support of the hammock. Stimulation of the ventral part of the midline area of the pons (P3 to P7, H-7.5 to H-9.5) increased the tone of the hindlimb muscles, with the force through the hindlimb and the tonic discharge of extensor muscles increasing with increased stimulus intensity. Such an increase of the force level continued after termination of the stimulation. When the dorsal pontine stimulation was followed by the ventral pontine one, the animal could regain the standing posture. Histological examination revealed that the depressant effect on the tone of hindlimb extensor muscles was elicited from the caudal part of the nucleus centralis superior, and the augmenting effect on the tone of hindlimb extensor muscles was elicited from the rostral part of the nucleus raphe magnus. Depending on the site and intensity of the stimulation, it was possible to set and reset the level of postural muscle tone in the decerebrate animal. The changes persist for long periods after cessation of the stimulus train, and these results provide a way of controlling the excitability of the spinal cord in the decrebrate animal.